The present invention generally relates to lubricant coatings and, more particularly, to an apparatus and method for lubricating foils for fluid bearings in high speed rotating machinery.
Fluid bearings generally comprise two relatively movable elements (i.e., a bearing and a shaft also called a journal). A predetermined spacing between the bearing and shaft is filled with a fluid such as air. Foils (or thin sheets of a compliant material) disposed in the bearing are deflected by the hydrodynamic film forces between the adjacent bearing surfaces. The foils thus enhance the hydrodynamic characteristics of the fluid bearing and also provide improved operation under extreme load conditions when normal bearing failure might otherwise occur. Additionally, these foils provide the added advantage of accommodating eccentricity of the relatively movable elements and further provide a cushioning and dampening effect.
To properly position the foils between the movable bearing elements, it has been common to mount a plurality of individually spaced foils on a foil bearing disk and position the disk around the shaft. Another common practice has been to provide separate compliant stiffener elements or springs beneath the foils to supply the required compliance.
In the operation of fluid bearings, there can be actual rubbing contact between the foils and the bearing surfaces of the shaft. To minimize friction and wear, the foils are typically coated with a lubricant. Typical coatings which incorporate solid lubricants use several polymers and several solid lubricants. The compounds comprising the lubricant have included fluorinated hydrocarbon polymer, graphite, and molybdenum disulfide. In order to adhere the foregoing compounds to a metal substrate, a binder has typically been added to the lubricant composition. For example, graphite fluoride with a polyimide resin binder has been used.
Yet, a polyimide composition that is suitable to bond to a substrate may not likely be suitable to act as a lubricant. Further, polyimide compositions with solid lubricants contain solvents. Such compositions cannot be applied as a thick layer due to the formation of bubbles and imperfections (e.g., segregation of solids into lumps) during a curing cycle. Otherwise, performance of the layer is compromised. Therefore, in U.S. Pat. No. 4,435,839, a multi-layered coating of a graphite-fluoride-polyimide mixture was applied to a roughened contact surface. After the application of each sub-coat, the contact surface and sub-coatings were cured. Also, the outer coating surface was de-roughened at predetermined thickness intervals during the application of the multiple sub-coatings to provide the needed surface smoothness and thickness uniformity. The final overall coating was cured and then a graphite fluoride powder was burnished into the final sub-coat. Yet, the multiple steps required in this process make the actual practice of the invention costly and time consuming.
Similar to U.S. Pat. No. 4,435,839, the 1972 National Aeronautics and Space Administration Technical Note no. NASA TN D-6714 entitled xe2x80x9cGraphite Fluoride as a Solid Lubricant in a Polyimide Binderxe2x80x9d discloses a method of roughening the contact surfaces and alternating the application of a thin coat of a mixture of polyimide and solid lubricant with baking the thin coat. After the desired multi-layered coating thickness was achieved, the coating was further cured. Again, the need for alternating thin layers and curing results in high manufacturing costs.
In addition to the problems of merely adhering a lubricant coating to a substrate, there are performance problems of the coating itself. In one example of addressing coating performance, a lubricant using a partially defluorinated graphite fluoride with a resin such as polyimide was provided in U.S. Pat. No. 4,500,678. The graphite fluoride was dispersed in a dispersion medium and then the mixture was subjected to electromagnetic radiation. The radiation effects partial degradation of the graphite fluoride such that the fluorine atoms were partially removed. The partial degradation was said to allow greater compatibility with resins, greases, and oils that may be mixed with the degraded graphite fluoride to produce the final lubricant. Disadvantages, however, to this invention include loss of fluorine, loss of lubricating properties, increased wetting, and degradation of coatings due to oil contamination.
In another instance of addressing performance, R. Fusaro, in xe2x80x9cPolyimides: Synthesis, Characterization and Applicationsxe2x80x9d, Vol. 2, edited by K. L. Mittal, Plenum Press, pp. 1053-80 (1984) describes the use of polyimide with either molybdenum disulfide and graphite fluoride. Fusaro suggested that ambient temperature and atmosphere affect the friction and film wear properties of the polyimide films. In particular, some polyimides possessed a transition temperature, above which the molecules obtained a degree of freedom in movement. Dry air appeared to lower the transition temperature, according to Fusaro. Thus, Fusaro sought to alter the polyimide by the addition of graphite fluoride, for example, to induce the shear formation of thin layers at temperatures below the transition to obtain optimum lubrication.
The problems of coating performance and bonding the coating to a substrate are illustrated in FIG. 1. Therein, graphite fluoride in a polyimide resin matrix was applied over a stainless steel substrate. The graphite fluoride quickly reacted with the stainless steel surface to form oxides and fluorides in a layer that is thick and porous. This problem of a brown layer forming under the coating is enhanced by the water produced by the polyimide during a high temperature cure. The resulting coating peels off or is easily scratched off. An Energy Dispersive X-ray Analysis (EDAX) chemical analysis of the brown layer in FIG. 1 shows the presence of Cr, Fe, O, and F, indicating the formation of fluorides and oxides of Fe and Cr (FIG. 2).
Additional related disclosures are found in U.S. Pat. Nos. 5,958,847; 5,560,283; 5,363,821; 5,257,603; 5,239,955; 5,137,751; 5,004,627; and 4,831,977.
As can be seen, there is a need for an improved lubricant coating for fluid bearings, as an example. Another need is for an improved method of making a lubricant coating. Also needed is a lubricant coating and method of making the same that provides improved bonding to a substrate. A lubricant coating and method of making the same is needed that provides better friction and wear properties at the same time, forming a better bond. A lubricant coating is needed that obviates the need for multiple thin layers and alternating curing steps, which otherwise leads to increased manufacturing time and costs. Further needed is a lubricant coating that provides chemical reactivity with the substrate on which the coating is applied to produce good bond quality between the coating and substrate.
In one aspect of the present invention, a lubricant coating disposed between a substrate and a counter surface comprises a reaction layer immediately adjacent the substrate; a bonding layer immediately adjacent the reaction layer, with the bonding layer comprising a first composition; and a low friction, lubricious layer immediately adjacent the bonding layer, with the lubricious layer comprising a second composition that is different from the first composition.
In another aspect of the present invention, a method of making a lubricant coating on a substrate comprises applying a bonding layer on the substrate, with the bonding layer comprising a first composition; heating the bonding layer in the substantial absence of curing the bonding layer; forming a reaction layer between the bonding layer and substrate; applying a lubricious layer on the bonding layer, with the lubricious layer comprising a second composition that is different from the first composition; and curing the bonding and lubricious layers.
These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims.